The invention relates to a keyboard, especially for electronic data-processing apparatuses.
A multiplicity of electronic apparatuses, such as visual display units, are controlled via keyboards. In general a keyboard consists of a separate keyboard housing which is arranged in front of the electronic apparatus and has one or more control panels which project out of the housing, such as, for example, a typewriter keyboard, in which individual keys or key switches, in each case arranged in columns and rows, are arranged. The individual keys or key switches themselves may be soldered onto a common board which is fitted with electronic components, especially microprocessors. Each key switch has allocated to it one or more meanings, such as, for example, letters, digits or the like, which are initiated by key operation. In order to differentiate between the individual keys, said keys are coded in, for example, binary signals. The key which has been operated is converted via, for example, a serial interface into a specific binary code, 8 to 10 binary bits in the form of square-wave pulses normally being used. In this bit pattern, the individual keys are unambiguously allocated with respect to one another on the basis of their function. The allocation with respect to one another in this case takes place by means of a microprocessor on the board.
The microprocessors operate, for example, at a clock frequency of between 1 and 6 MHz, that is to say, at 6 MHz, and a switching process takes place 6 times per microsecond. These switching processes, which run at a high switching speed, cause radio-frequency interference which triggers the computer or the microprocessor. A spectrum of interference frequencies is accordingly produced which frequencies, depending on the coding and the system of key interrogation, contain information which can be allocated unambiguously to the character entered on the keyboard. This interference is, on the one hand, radiated directly into the air and, on the other hand, is emitted to the environment via the keyboard cable and the computer connected thereto, as conducted interference.
A certain amount of shielding of the board holding the individual key switches is achieved by providing a metal frame, constructed from flat sheet metal, in which the housings of the key switches are inserted. In consequence, a reduction in the interference level values is already achieved. However, in practice, radio-frequency interference can still occur, as in the past, to such a severe extent that it is possible to intercept these frequencies. This can lead to undesired detection of data and information by unauthorized personnel.
A housing for electronic apparatuses has been disclosed in DE 20 32 408 B 2, whose electromagnetic radiation to the outside is shielded and which is shielded against electromagnetic radiation from the outside. This is done by means of two shell parts, which are produced from plastic, are plugged one inside the other and form a type of double wall. In this arrangement, the inner shell part is provided with a metal surface.
This known embodiment may achieve a significant reduction in the electromagnetic radiation which possibly emerges to the outside. However, it is extraordinarily expensive. Furthermore, the consequent implementation of the radiation shielding in the region of the key guide has not been continued.
GB 1,550,758 has likewise disclosed a keyboard arrangement whose electronic circuits are intended to be protected against static electrical discharges. For this purpose, comprehensive measures are provided for grounding, that is to say current dissipation. This document does not mention any consideration of RF radiation.
DE OS 39 18 951 A 1 has disclosed a shielding housing for a general application, in which housing the emergence of radio-frequency radiation is prevented, or at least reduced, by sharply-angled deflection edges being provided for the RF radiation. The emergence of radio-frequency radiation from a housing is accordingly prevented by a sharply-edged connection or joint or by means of a plurality of abrupt angles. In this arrangement, the intermediate spaces between the housing parts must be kept as small as possible. An electrically conductive material such as aluminum, brass or copper is proposed as the material for the known shielding housing.